1. Field of the Invention
The present invention relates to a liquid crystal reflective display which reflects incident rays by a liquid crystal to perform display.
2. Description of the Related Art
Various types of liquid crystal display devices have been proposed. For example, TFT liquid crystals including thin-film transistors for respective pixels are now available. Although the TFT liquid crystals can perform high-resolution display, they require precise and complicated manufacturing steps, and further suffer from low yields and therefore high costs.
Therefore, attention has been given to polymer dispersion liquid crystal display devices employing a liquid crystal and polymer composite film in which liquid crystal is dispersed in a polymer material, because the devices can be manufactured relatively easily. For example, U.S. Pat. No. 3,578,844 has disclosed a liquid crystal display device having a composite film of liquid crystal and polymer, in which cholesteric liquid crystal is used and dispersed in a polymer material.
In this specification, a side opposed to a surface, on which a liquid crystal reflective display displays an image, is referred to as an observation side.
The cholesteric liquid crystal contained in the polymer dispersion liquid crystal display device has a memory effect to attain stably two states even while a voltage is not being applied. Therefore, high-resolution display can be performed only by simple matrix drive without using complicated circuits provided with active elements such as TFT liquid crystal.
Examples of the polymer dispersion liquid crystal reflective displays of the above type are disclosed in U. S. Pat. No. 5,200,845 and Japanese Tokuhyohei No. 6-507505 (JP, A, No. 6-507505). These liquid crystal display devices perform color display with a colored transparent state attained by selective reflection by a cholesteric phase and a scattered state thereof. This is achieved by reflective display performed in such a manner that the chiral nematic liquid crystal selectively attains the light transmission state, in which helical axes attain a random focal conic state owing to application of a low-voltage pulse, and the selective reflection state, in which helical axes are aligned to attain a planar state owing to application of a high-voltage pulse. In LIQUID CRYSTALS, 1992, Vol. 12, No. 1, pp. 49-58, such a structure is disclosed that liquid crystal display layers, of which selective reflection wavelengths correspond to red, green and blue light, respectively, are layered, and simple matrix drive of each display layer is performed independently of the other layers, so that reflective display rays of red, green and blue are produced.
However, the liquid crystal reflective display devices which have been proposed cannot perform display at a sufficiently high quality. In particular, it has been found from the study by the inventors that, as the selective reflection wavelength of liquid crystal increases, the transparency in the transparent state as well as the clarity of the displayed color in the selective reflection state are impaired to a higher extent, which impairs the display quality.
The selective reflection wavelengths of the cholesteric liquid crystal have a viewing angle dependency, and the displayed color changes when viewed obliquely. Therefore, good white display cannot be performed, when all the layers i.e., red, green and blue display layers are simultaneously set to the reflection state in the above display device having the three-layer structure.
As described above, a liquid crystal display capable of good white and color display has not been available.